WEAK, FEEBLE, FRAIL, FRAGILE, INFIRM, DECREPIT mean not strong enough to endure strain, pressure, or strenuous effort.

WEAK applies to deficiency or inferiority in strength or power of any sort.

e.g. felt weak after the surgery

FEEBLE suggests extreme weakness inviting pity or contempt.

e.g. a feeble attempt to walk

FRAIL implies delicacy and slightness of constitution or structure.

e.g. a frail teenager unable to enjoy sports

FRAGILE suggests frailty and brittleness unable to resist rough usage.

e.g. a reclusive poet too fragile for the rigors of this world

INFIRM suggests instability, unsoundness, and insecurity due to old age or crippling illness.

e.g. infirm residents requiring constant care

DECREPIT implies being worn-out or broken-down from long use or old age.

e.g. the dowager's decrepit retainers

adjective (feebler, feeblest)

lacking physical strength, especially as a result of age or illness:

〈人･体の一部が〉(老齢･病気などで非常に)弱った, 衰弱した; 力のない (!weakよりも強意的な語; 哀れみや軽蔑を含意する) .

e.g. my legs are very feeble after the flu.

(of a sound) faint:

〈音･光などが〉微弱な, かすかな.

e.g. his voice sounded feeble and far away.

lacking strength of character:

〈力･勢力･効果などが〉弱い, 不足している, 不十分な

e.g. she overreacted in such a feeble, juvenile way.

failing to convince or impress:

〈冗談･言い訳などが〉効果のない, 説得力を欠く

e.g. a feeble excuse.

feebleness |ˈfēbəlnəs| noun

Middle English: from Old French fieble, earlier fleible, from Latin flebilis ‘lamentable’, from flere ‘weep’.